Recovery of ethane and gasoline from natural gas



Ont 7, 1947 J. w. LA-rcHuM, JR 2,423,521

RECOVERY OP ETHANB AND GASOLINE PROv NATURAL GAS sued Dee. 1v, 1945.

ATTORNEYS Patented ocr. 1, 1947A RECOVERY OF ETHANE AND GASOLINE FROMNATURAL GAS John W. Latchum, Jr., Bartlesville, Okla., assignor toPhillips Petroleum Company, a corporation of Delaware ApplicationDecember 17, 1945, Serial No. 635,589

3 Claims. l

This invention relates to the treatment of hydrocarbons. In one of itsmore specific aspects it relates to a method for the separation andrecovery of ethane from a natural hydrocarbon gas.

In a still more limited aspect it consistsprimarily of a process for theelimination of methane from ethane and heavier constituents present inan absorption plants rich oil.

In the treatment of natural gas for the separation and recovery ofindividual hydrocarbons many problems exist. Some of the more diiiicultproblems lencountered are those involved in the separation and isolationof such normally gaseous hydrocarbons as ethane or propane from anatural gas or mixture of relatively volatile hydrocarbons containingnormally gaseous constituents. It is to the separation and recovery ofan ethane product of suiiicient purity for many commercial uses that myinvention is directed.

One object of my invention is to provide a process for the separationand recovery of an ethane product from a natural gas or otherhydrocarbon gaseous mixture containing ethane.

Another object of my invention is to provide a process for thesimultaneous separation of ethane and natural gasoline from natural gas.

Still another object of my invention is to provide a process for thesubstantially complete elimination of methane from the ethane andnatural gasoline products during separation of these from natural gas.

Still other objects andv advantages will be obvious to those skilled inthe art from a careful study of the following disclosure and theattached drawing which respectively describes and illustrates apreferred embodiment of my invention.

The drawing is a schematic now diagram of a preferred embodiment of myunitaryseparation system.

Referring now to the drawing, numerals l and 2 refer to absorptioncolumns while fractionational distillation columns are identified bynumerals 3 and 3A. A drier unit 4 and a cooler 5 are installed in a gasline 6 between the absorption columns l and 2 as indicated. Followingthe absorber 2 are some ash tanks 1, 8, 9 and I0. The fractionator 3 hasan overhead vapor line Il which conveys overhead vapors to a condenserl2 and continues on to a reux accumulator tank I3.

All pressures stated hereinafter are in terms of pounds per square inchabsolute'.

In the operation of this equipment according to my process a distillatewell iiuid 'or natural or hydrocarbon gasl from anyv other source at apressure of about 1,000 pounds per square inch and at about F.,enters'my system by way of a raw gas line 2|. This line leads the gas orfluid to be treated to the first absorber vessel I at about the lattersmidpoint. This absorber operates at substantially the pressure andtemperature of the inlet gas. An absorption oil, to be describedsubsequently, enters the absorber by way of a lean oil line 22 and afterbecoming enriched with dissolved hydrocarbons leaves the absorber by wayof a line 23 and is then termed ,rich or fat oil. Gas stripped of itscondensible hydrocarbon content at the temperature and pressure of thisabsorber leaves this vessel through the overhead gas line E. Enteringthe tower at a point in the base is a line 24 which carries a dry ormoisturefree gas from a source to be explained hereinafter.

In a subsequent step for the absorption of the more volatilehydrocarbons including ethane, the operation is preferably carried outat a relatively low temperature in order to increase the solubility ofthe gaseous ethane in the absorption oil. In order to carry on anoperation, such as an absorption step, at temperatures below thefreezing point of water, the absorption oil and the gas to be treatedshould preferably be relatively dry or free, from moisture. Theabsorption oil should be free from suspended water so that upon coolingor chilling to a temperature below the freezing point of water dropletsof ice will not be present. Similarly the gas entering such a stepshould also be dry since its moisture content would tend to freeze out,`during the contacting. A further reason for drying the gas to be treatedis to prevent hydrate formations in the chiller and pipes leading fromthe chiller to the absorption vessel. l

Substantially any type of drying apparatus desired may be usedproviding, of course, that it is capable of treating a relatively largevolume of lgas to the required degree of dryness. I have found that abauxite dehydratorworks well for this purpose. Since some well gasescontain appreciable moisture, I have found it advisableto have more thanone, for example, two, bauxite dehydrator vessels and to use themalternately; when one is on process, the other is being regenerated ordried.

In order to be able to use the very efiicient adsorptionk type ofdehydration catalyst, for best operation I have found the hereinbeforementioned absorption step should be used. This abf sorption step carriedout at about atmospheric or normal absorption operating temperature isintended mainly'to remove the most easily condensible, relatively highboiling hydrocarbons from the gas stream. Said hydrocarbons are easilyadsorbed by such adsorbents as the bauxite dehydration adsorbent ofvessel 4, and these hy- 5 drocarbons tend to poison or to makeineffective the bauxite for moisture removal. Thus, in order to providefor a long operating life of the bauxite dehydration catalyst in vessel4, I prefer to use an absorption unit or as it might be termed aprea-bsorber to remove the heavy ends from the gas stream prior to thedehydration step. These heavy ends usually comprise aromatic andnaphthenic hydrocarbons and on occasion other high carbon to hydrogenratio compounds such as asphaltenes. I

From the dehydrating zone 4 the dry gas passes through the cooler 5which is adapted to chill the gas to about 30 F. At this temperature thechilled gas is passed into the second absorber vessel 2 which isoperated at about the chilled gas inlet temperature. Pressure in thisvessel is about the same as in absorber i, being lower only by thepressure drop experienced in flowing through the line 6, drier 4 andcooler 5. The gas enters this absorber at a point near the bottom,passes upward through the contactors within the vessel and iinallyleaves through the overhead residue gas line 25 for such disposal asdesired.

While I have not shown it in the drawing, this cold, dry gas may pass inindirect heat exchange with the dry Warm gas prior to passage of thelatter through the chiller 5 to assist in reducing refrigeration costs.

Lean absorption oil enters this absorber 2 by 35 Way oi' a lean 'oilinlet line 26 which discharges the oil upon a top contacting tray. Theline 26 carries a refrigeration means 21 for cooling the absorption oilto any desired low temperature, for example, to the 30 F. temperature ofthis 40 absorber. I'n this absorber the relatively Volatile liquidhydrocarbons and the normally gaseoushydrocarbons,excepting-for-themost-part the methanee-are-absorbed and carried out asenriched absorption oil or fat oil. 'I'his enriched 45 absorbent passesdthrough the rich oil line 28 and enters the lower portion/of thev flashtank 1. Just prior to entry into this first flash tank 1 this fat oilstream is joined by the fat oil stream from the base of the rst absorberl. This latter stream flows from its absorber through line 23 and line29 to join the second fat oil stream from absorber 2 in line 28 justafter passage through a pressure reducer 38. This first ilash tank isoperated at about 500 pounds pressure at about 14 F. This temperatureresults from the mixing of the warm rich oil stream from absorber l andthe cold stream from absorber 2 and the mixture passingthrough thepressure reducer 30. At this pressure in ilash tank 1 a 60 considerableproportion of the dissolved methane ashes from the absorption oil andpasses out rby way of a gas line 3|. This vented oil leaves the rstflasher through a line 32 containing a pressure reducer 33 and enters asecond vent 65 tank 8 at a pressure of about 250 pounds. Evolution of afurther quantity of dissolved methane cools the absorption oil to about17 F., the freed gas passing out through a vent line 34 to join the gasvented from tank 1 in the line 24. 70 This further vented oil passesfrom the tank 8 through an oil line 35 and pressure reducing valve 36and enters a third vent tank 9. This tank is operated at about poundspressure and about 19 F. and the oil on passing through the 75 valve 38loses a still further quantity of dissolved methane. since the partialpressure of ethane at the operating conditions of tank 9 is appreciable.In addition the vent gases from tanks 8 and 1 also carry some ethane butcorrespondingly less as their operating pressure is higher than'that in'tank 9. Thus the vent gas from tank 3 leaves by way of a. vent line 31to join the vent gases from tanks 8 and 1 in line 24.

Since this mixture of vent gases contains some ethane and ethane is oneoi the desired products of the process, this gas is recycled to the rstabsorber by suitable recompression means, not

shown. This gas is also dry with respect to moisture and being dry it ispassed into the absorber i at a point in or near the bottom and thenpassed upward through the entire length of the column. In this mannersome little ethane is probably absorbed but the main function of thisdry gas in this first absorber is to dry the absorption oil in thelatters passage through the bottom half or drier section of the vessel.The oil needs to be dry or free from moisture to prevent ice andgas-hydrate formation in the vent I have found that by operatingsubstantially the lower half of vessel I as a moisture stripper, thatthe rich absorbent oil issuing therefrom is suiiiciently dry to cause nomoisture freezing or hydrate formation in the vent tank 1.

The moisture removed from the absorbent oil in the lower half of theabsorber I combined with the moisture coming in with the raw gas to betreated passes overhead from this vessel and is removed from the gasstream by dehydration zone 4. Thus, the gas feed entering the lowtemperature absorber 2 is thoroughly dried and accordingly the rich oilleaving this absorber is dry. In this manner all inlet material to thesequence of vent tanks is free of moisture.

The three-time vented oil leaves tank 9 through lan oil lineV 38 andpasses through still another pressure reducing valve 39 which lowers thepressure to about 30 pounds per square inch (absolute) and enters thefinal vent tank I0. There is substantially no net temperature dropaccompanying this expansion since heat absorption from the atmosphereoffsets any decrease due to the expansion. In this tank a largeVproportion of the gas vented is ethane, a product of the process, andthis vent gas passes through a vent line 40, is compressed by acompressor 4l and at a pressure of about 365 pounds per square inch'passes on through line 40 extended to the approximate center' of asecond fractionator column 3A. Any methane contained in this gaseousstream will be included in the nal ethane concentrate product since nofurther'separation is herein contemplated. I have found that this amountof methane can be materially decreased by withdrawing a portion of thevent gas from line 40 and passing it through a recycle gas line 43 tojoin th'e feed to the several vent tanks 1, 8, 9 and I0. In this mannera further opportunity is at hand for venting some of this methane.

The fully vented absorption oil from the inal vent tank I0 passestherefrom through a. line 42 and is forced into the fractionator 3 by apump 5I under a pressure of about 80 pounds per square inch. A reboilercoil 44 furnishes suiiicient reboiling heat as to maintain a kettletemperature of about 350 F. This temperature I have found is ample tomake a substantially complete separation between hydrocarbons normallyincluded within a natural gasoline boilingrange and the 'I'his methanecarries some ethane,

absorption oil. Accordingly, the absorption oil is withdrawn from thebase oi this iractionator through a lean oil line I9 and is fractionatedin column B1 to remove tarry matter and overhead lean oil is nallycooled in a cooler 60 to about 90 l". at which temperature the lean oilis suitable for recycling into the ilrst absorber column. Overheadvapors are taken from this column 3 at about 180 F., are cooled in thecondenser l2 and enough condensate formed to supply reflux necessary forcooling the top of the tower. Th'e liquid which accumulates inaccumulator I3 is pumped through a reux line 45 into the top of thefraotion'ator. The uncondensed overhead gases which accumulate in thevessel I3 pass by way of a vapor line 46 and are added to the ethanerich gas in line lill.y The mixture then'is pumped by compressor 4I onthrough line 40 extended into about the midportion of the secondfractionator 3A.

The fractionator 3A is operated at a pressure o about 360 pounds persquare inch. A reboiler coil e2 furnishes sumcient heat to maintain akettle temperature of about 220 F. Under these conditions ethane isdriven overhead as a vapor through a line lid.' This ethane is cooled ina cooler 53 sumcient to produce reflux liquid which accumulates in thevessel 5d. The liquid is pumped by a pump E5 through a reflux line et tothe top of this tower for reiluxing purposes. Gaseous ethane is takenfrom the system through a product line 5l for such disposal as desired.

Bottoms from this latter fractionator are removed through a bottoms linedl and sent to a storage vessel, not shown, or to such other disposal asdesired.

In case the well emuent being treated accord- 1o through a bottoms line58 and passed to such disposal as desired. Overhead vapors are condensedinl a cooler 50, the condensate accumulating in vessel 60. That requiredfor refluxing is pumped through reflux line El to the top of the towerwhile the main purified stream passes through line 22, cooler 50 on toabsorber l and to chiller 2l and absorber 2.

The several coolers and/or condensers used herein may be merely watercooled heat exchangers, or may be refrigerated chillers, de-

pending upon the service required.

The absorption oil may be any desired type of oil, as, for example, astraight run parafllnic type oil boiling from approximately 200 F. to350 F.,

or may be a higher boiling or lower boiling oil as desired. That is, theparticular oil used is not necessarily critical.

The following tabulation gives an example of the operation of theprocess of my invention when 3@ applied to the extraction of condensiblehydrocarbons and an ethane product fr m a distillate well productionfluid available at a pressure of about 1,000 pounds per square inch andat about atmospheric pressure.I The numerical values are pound mols perday of operation when treating 44 million standard cubic feet of Welleiliuent per day.

Hydrocan Gas Feed T0- Residue bon Com- Line 24 Gas Valve 30 Line 31Ponent Absorber 1 Absorber 2 Lme 25 ing to my process or other chargestock contains then passed on through the line 22, through cool- Themain function of the absorber I is to remove the heavy ends from the gaswhich is to be dried prior to the low temperature absorption step. Ifthe heavy ends of the gas were not removed, the bauxite dehydrator 4would become 5'5 poisoned by adsorption of said heavy ends. Thispoisoning interferes seriously with the dehydrating eihciency of bauxiteand I prefer to vhave all stocks entering the low temperature absorber2' as dry as possible.

Compres- Line 37 sor 41 'Abt Cunal er l0 andon to absorbers I and 2 ashereinbeforedescribed. v Itis preferable to distilv continuously thisab- From a studyof the above vtiilulation it will be lobserved that agaseous or fluid feed stock containing a normal amount of ethane andnatural isorption oil since any tarry material might tend` 15 gasolineboiling. range hydrocarbons may be emamasar ciently separated into anethane stock and a natural gasoline. An ethane concentrate such as `Iherein produce should be suitable for cracking to ethylene for themanufacture of ethyl alcohol. One unique point of my process, andmentioned hereinbefore, is the recycling of the combined dry It will beobvious to those skilled in such artA that many alterations andmodifications of myl process may be made, for example recycling thevented vapor from each iiash tank to the preceding ilashtank or makingthe temperature level the same or even ascending in place of descendingas shown.

For simplicity purposes most valves, pressure regulators and temperaturemeasuring and recording devices and such other auxiliary equipmentnormally used in such operations have not been expressly shown ordescribed, but the use of these items is well known to those skilled inthe art.

Materials of construction may b e selected from among those commerciallyavailable since s pecial materials or specially constructed vessels orpipes, etc., are not needed. y

The operational details given, such as temperatures, pressures and thelike, may be varied by those skilled in the art and yet obtain resultssimilar to those herein given. For example, the raw gas 'may beavailable at a pressure other than 1,000 pounds per square inch, eitherhigher or lower, and under such conditions then the pressures, etc. atsome other points of my process may be varied accordingly and yet obtainabout the same overall result. For example, if feed gas is avialable atsay 1,200 pounds pressure, both absorbers may be operatedgat about thispressure, and the vent tank 1 may be operated at 500 to 700 pounds, andthe other vent ta'nks at corresponding pressures. Likewise the twofractionator towers 3 and 4 may be operated at the pressures andtemperatures given or may be varied some.

Having disclosed my invention, I claim:

1. A process for treating a natural hydrocarbon gas for the separationof an ethane product and a natural gasoline product comprising the stepsof contacting the gas with an absorption oil at approximatelyatmospheric temperature to produce a treated-gas substantially free ofhigh boiling natural gasoline boiling range hydrocarbons and containingsome moisture and a rich absorption oil containing said last mentionedhyzone separating an ethane concentrate 'and a natural gasoline productas products of the -process and recycling a portion of the remainingabsorption oil as the rst mentioned absorption oil,- chilling theremaining portion of the absorption oil and recycling this chilledabsorption oil as' the rst mentioned chilled absorption oil; andremoving the ethane concentrate and the naturall gasoline product as themain products of the process.

2. The method of claim 1 wherein the vented' gas substantially lighterthan the desired ethane l is contacted with the first mentioned. richabstreams.

3. A processv for producing a moisture free` ethane concentrate and amoisture free natural gasoline product from a distillate well eflluentcontaining some moisture comprising passing the well eflluent'containingsome moisture into the mid section of a contacting zone having an inletend and an outlet end with respect to albsorption oil ow, introducingabsorption oil into the inlet- `bined stream of well effluent gas andvent gas containing moisture from the inlet end of said contactingzoneand contacting this combined gas stream with a dehydrating adsorbent,chilling the so contacted gas stream and passing the chilled 'gas intoan absorption zone in contact with a' chilled absorption oil forabsorption of ethane and natural gasoline boiling range hydrocarbons,removing contacted gas as a residue gas product of the process,combining said chilled absorption oil containing said ethane and naturalgasoline boiling range hydrocarbons and aforesaid dry rich absorptionoil and passing said combined oils to a venting zone and ventingtherefromV dry dissolved gas substantially lighter than the desiredethane and recycling same to the first-mentioned contacting zone as saidsubsequently produced dry vent gas; passing the so vented absorption oilto a distillation zone and removing therefrom an ethane'concentrate anda natural gasoline product as the main products -of theprocess, and anabsorption oil bottoms, and recycling a portion of this absorption oilto the rst stated contacting zone, chilling the remaining drocarbons,treating this contacted gas stream.

of the process;l combining the two rich absorption oils and passing thecombined oils to a venting zone, venting dissolved gas substantiallylighter than the desired ethane and removing the vented absorption oilfrom said venting zone and passing,

same to a fractionation zone, from this latter portion and recyclingthis chilled portion as the rst mentioned chilled absorption oil.

JOHN W. LATCHUM, Jn. REFERENCES crrEp The following references are ofrecord in the' file of this patent:

UNITED STATES PATENTS

